Harmonic trammel drive

ABSTRACT

Improvements to trammel type drives used to convert rotary motion to linear motion in which the dwells at the ends of the linear motion are increased and in which the kinematic characteristics of the linear output motion may be significantly altered from the conventional harmonic.

States atent [191 [111 r 3,869,925 Mar. 11, 1975 HARMONIC TRAMMEL DRIVE [76] Inventor: John Henry Brems, 32867 White Oaks Trail, Birmingham, Mich. 48010 [22] Filed: July 16, 1973 [21] Appl. No.: 379,375

[52] US. Cl. 74/25 [51] Int. Cl. Fl6h 21/16 [58] Field of Search 74/62, 25

[ 56] References Cited UNITED STATES PATENTS 442,021 12/1890 Davenport et al. 74/25 823,046 6/1906 Johanson 74/25 1,113,641 10/1914 Johanson 74/62 2,708,848 5/1955 Hohenner 74/25 2,932,785 4/1960 Shovic 74/25 5/1962 Debeh ..74/62 6/1965 Danon ..74/25 OTHER PUBLICATIONS Schwamb, Merrill, James; Elements of Mechanisms, pages 134-141, Sixth Edition, July, 1947.

Primary Examiner-Wesley S. Ratliff, Jr. Attorney, Agent, or FirmBarnes, Kisselle, Raisch &

- Choate 5 7 ABSTRACT Improvements to trammel type drives used to convert rotary motion to linear motion in which the dwells at the ends of the linear motion are increased and in which the kinematic characteristics of the linear output motion may be significantly altered from the conventional harmonic.

10 Claims, 12 Drawing Figures PATENTED 3.869.925

sum 3 o g PATENTED 1 I975 mnumg m u ON HARMONIC TRAMMEL DRIVE This invention relates to a trammel drive and more particularly to certain modifying improvements.

In mechanisms for developing a reciprocating motion, it has been difficult to overcome a practical deficiency, namely, that the dwell at the end of a stroke is very short and, if the drive crank is not accurately positioned upon stopping, the output position cannot be accurately maintained. This is an existing fault in all harmonic drives such as cranks, scotch yokes, etc.

It is an object of the present invention to provide a reciprocating power drive which will not only lengthen the dwell of the linear motion of the output, thus rendering the stopping point of the crank element much less critical, but will also modify the acceleration and deceleration characteristics of the output to make it a more desirable means to provide the desired motion.

Other objects and features of the invention will be apparent in the following description and drawings in which are described and illustrated the principles of operation of the invention together with use in the best mode presently contemplated for the practice of the invention.

Drawings accompany the disclosure and the various views thereof may be briefly described as:

FIG. I, a partially fragmented isometric diagrammatic view of a commonly used harmonic trammel drive.

FIG. 2, a diagrammatic view of an improvement of a harmonic trammel drive. 1

FIG. 3, an acceleration and velocity graph representation of the motion ofthe structure illustrated in FIG. 2.

FIG. 4, a diagrammatic view of a modification of the improvement of a trammel drive.

FIG. 5, a diagrammatic view of a still further modification of the improvement of a trammel drive.

FIG. 6, a kinematic Iine diagram of the invention.

FIG. 7, a velocity graph of the mechanism illustrated in FIG. 6.

FIG. 8, a kinematic line diagram of another version of the invention.

FIG. 9, a velocity graph of the mechanism illustrated in FIG. 8.

FIG. 10, a kinematic line diagram of still another version of the invention.

FIG. II, a velocity graph of the mechanism illustrated in FIG. 10.

FIG. I2, a further modification of the mechanism with an inverted guide system.

In FIG. 1, crank arm is rotated by the output shaft 12 of speed reducing assembly 14 having an input shaft 16 and is driven by a motor 18 through a coupler 20. On the other end of input shaft 16 is coupled a brake assembly 22. Crank arm 10 is pivotally connected to lever 24 by pin 26 and spacer 28. Reciprocating bar 30 is guided to travel in a straight line by support rollers 32 and is joined to the load to be reciprocated, which is not shown, by slot 34 and pin 36. Lever 24 is pivotally attached to bar 30 by pin 38 and spacer 40. On the other end of lever 24, roller 42 is carried by pin 44. This straightforward design described in many mechanism books directs that the center distance of the holes in crank 10 be exactly the same distance as that be tween the holes for the pin 26 and pin 38 and also pin 26 and pin 44 in lever 24'. When lever 10 rotates towards the position shown at 10a, the roller 42 travels straight down and will enter the slot 46 in guide member 48 and will compel it to continue on a straight line while it passes through bottom dead center and back up again until the kinematics of the arrangement will again guide it in a straight line back to center. When the crank 10 has traveled l80, as shown at 10b, the travel of bar 30 will be exactly four times the distance as the space between the center of the holes in crank 10.

This crank travel must be exactly and exactly from horizontal to horizontal. wherein lies the problem with this classic harmonic trammel drive and all harmonic devices, such as cranks and scotch yokes.- In this case, ifcrank I0 fails to reach true horizontal on either end of its rotation, the travel of bar 30 is short. If it goes beyond the horizontal, full stroke will be made and then reverse itself by some amount. True position is normally attempted by electrically sensing the position of the crank 10 and signaling the motor 18 to stop and brake 22 to engage. In some applications, mechanical stops are used to limit the rotation of crank 10 to 180. It can readily be seen that any electrical control malfunction with or without stops would cause problems of a serious nature. The harmonic acceleration and velocity curves of this mechanism are shown in FIG. 3 by the solid curves. s

Shown in FIG. 2 is the same linkage as FIG. I with the following changes and additions. Crank 10 of FIG. 1 is replaced by crank 52 which has the hole provided for pin 26 replaced by a slot 50 in crank 52 of FIG. 2. Lever 24 in FIG. 1 has added to it a second roller 54 secured by pin 56 arbitrarily located approximately mid-point between pin holes 26 and 44 in FIG. 2. It should be noted that the position of the roller 54 is not critical and may be placed anywhere on the lever 24 within reason, including being made coincidental with the pin 26. Slot 46 and guide 48 of FIG. I have been extended as slot 46a and guide 48a up to the center of the crank shaft 12 in FIG. 2. A radius slot 58 swung about the center of pin 38 when bar 30 is at one end of its stroke and defined by guides 60 which extend down to a horizontal line through the crank shaft 12 and are so arranged to cooperate with the new roller 54 at the same time that roller 42 leaves its guide slot 46a, thus allowing crank 52 to overtravel without affecting thedwell position of bar 30. Slot 50 will accommodate any reasonable displacement of pin 26 during this overtravel. It should be noted that slot 50 need not have a closed outer end but may be open as shown by slot 53 in crank arm 55 in FIG. 4. This would allow pin 26 to become completely disengaged from the slot and allow crank arm 55 to revolve a full revolution or more without severe damage to the mechanism in case of any control failure.

In FIG. 2, when the crank reverses to make a full stroke of bar 30, no motion of the bar will occur until roller 54 leaves the guide slot 58 and roller 42 enters the guide slot 46a which will control the motion for exactly 180 of the crank arm 52 travel. At this time, roller 54 will enter into an identical slot as 58 at 62 which is formed by a radius swung about the center of pin 38 on this opposite end o'f'the stroke. This is fine as far as movement is concerned, but there is a discontinuity in the acceleration curve exactly like the classic drive as shown by the solid curves in FIG. 3. This is acceptable. but undesirable. All geneva mechanisms exhibit the same characteristics. This fault can be corrected as will be shown in further modifications. It is apparent that by slotting crank 52 all the reciprocating loads are removed from the crank arm and the crank shaft bearings which now are subject to pure torque loads only.

A method to-overcome the discontinuity in the acceleration characteristics is shown in FIG. 4. In this design, the guide member 48b containing slot 46b has been lowered from its position as shown in FIG. 2 and the auxiliary guide bars 66 added to form slot 64 cooperating with roller 54. Slot 64 is constructed such that at the transition, i.e., at that point where roller 42 leaves slot 46b and roller 54 enters slot 64, the velocity and acceleration generated by both roller slot combinations is the same. This is shown as point T on the acceleration and velocity curves of FIG. 3. Asthe roller 54 traverses that section of slot 64 labeled B, the aceeleration and velocity of the output are'shown by the dotted curves in section D in FIG. 3. In practical design, the desired transition point is chosen and section D of the acceleration curve drawn in. Then by numerical integration, the section D of the velocity curve is obtained, and by subsequent numerical integration, the geometric characteristics of section B of slot 64 are obtained.

The section A of the slot 64 is again a true are about the center of pin 38a at the end of the stroke, and a true dwell of theflitput is achieved over a wide range of angular positions of arm 24.

The modifications described in FIGS. 2 and 4 concern themselves with the creation of a true extended dwell at the ends of a stroke, while retaining substantially harmonic kinematic characteristics during the center portions of the movement.

A further important modification of a basic trammel in which the normal harmonic kinematic characteristies are significantly altered, is shown in FIG. 5. Pin 26 need not be in the center of the lever 24 but may be displaced thcrefrom by some reasonable amount; this permits a significant control ofthe dynamic characteristics during the major center portion of a stroke.

To illustrate the possibilities, reference is made to the line kinematic sketch in FIG. 6. It will be understood that the pin 38 is guided for movement along the line ZZ, and the roller 42 is guided in slot 46 perpendicular to line 22. Ifx and y are the coordinates to the centerline of thepin 26, it can be shown that the path of the movement is With the pin 26 driven by the slot 50 in arm 52, at any angle Y/X tan 6 Combining l and 2 to eliminate I, it can be shown X B/[B /A tan 0] Ifwe defineA +B=L and E=B/(A +B)=B/L, then A (l E) L and B EL. Substituting into equation (3) and simplifying X EL/ltE/l E)? 01"? To establish the movement U, from one end of the stroke when 6 0 U L S and S X l/l E Therefore By differentiation, the velocity and acceleration are found to be:

where M l E/E d U/d0 M L C059 I 2 (M I) Sin 6/[C0s 6 (I E/E) Sin 0] Acceleration The velocity characteristics for several illustrative values of E are presented in FIG. 7. When E .5, the pin 26 lies exactly midway between pins 44 and 38 and the normal velocity profile results; when E .4 and pin 26 lies closer to pin 38 than pin 44, the midpoint velocity is reduced as shown; conversely, when E .6 and pin 26 lies closer to pin 44 than pin 38, the midpoint velocity is increased as shown. It can be seen that by a judicious choice of the value E, a wide range of velocity characteristics may be selected.

In addition to the flexibility provided by a shift of the pin 26 along the centerline between pins 44 and 38, it is possible to move the pin 26 transversely away from this centerline by some reasonable, amount defined as E,. This creates a nonsymmetry of the characteristics about the midpoint point of motion. This situation is shown in the kinematic diagram of FIG. 8. The equations of motion are of a complex transcendental nature that lend themselves to solution most easily by iterative techniques made practical by a programmable calculator or computer.

The velocity characteristics for several illustrative values of E, are presented in FIG. 9, the value of E being .45 for all three curves. It will be noted that when E .45 and E, O, the velocity curve is very flat during the center portion of the movement; when E .05, the center portion of movement experiences a gradually and uniformly increasing output velocity; when E .1, this same'effect is increased and a marked velocity increase is found near the end of the stroke. In essence, therefore, it becomes possible to tailor the nonsymmetry of the output dynamic characteristics over a wide range by a judicious choice of E.

A second and different means to control the nonsymmetry of the output characteristics is shown in FIG. 10. In this case, the slot 50 in arm 52 is inclined at some angle which is controlled by the parameter E the distance between the extension of the slot 50 centerline and the axis of shaft 12.

The velocity characteristics for several illustrative values of E are presented in FIG. 11, the value of E again being .45 for all three curves. While the general behavior of the parameter E is comparable to that of E slight differences exist which may be meaningful in some applications.

By designing the slot 50 in arm 52 with a curvature, uniform or variable, a still higher degree of kinematic flexibility may be achieved.

Summarizing, the addition of an auxiliary roller and mating slots to a trammel mechanism permits a meaningful extension of the dwell at the ends of the stroke and, if desired, elimination ofthe abrupt step in the ac celeration characteristics. Furthermore, the utilization of a slot in the driver arm permits a variation in drive pin 26 position along the longitudinal centerline of the link 24, or transversely to said centerline which creates a significant kinematic flexibility for the center portions of the stroke. It is obvious that either of these improvements may be incorporated singly or in combination.

In the previous embodiments, the control of lever 24 near the ends of the stroke is accomplished by a roller 54 mounted on the lever 24 engaging a slot formed by suitable guides such as 60 mounted to the frame. For mechanical or structural convenience, it is possible to invert this guide system, i.e., mount the roller to the frame while mounting the guides to the lever.

Such an arrangement is shown in FIG. 12. Two rollers 72 are suitably attached to the frame structure, and slot 74 is formed by two guides 76 attached to the lever 24. The rollers 72 and slot 74 perform the same guid ance for the lever 24 as was performed by the roller 54 and associated slots in the previous embodiments.

In all ofthe modifying structures described above, whether to increase the end of stroke dwell or whether to modify the otherwise harmonic kinematic characteristics, it was necessary to slot the crank 52 and 55 to accommodate the non-circular path of the pin 26. This, however, does not introduce lost motion or positive ambiguity into any of the systems. For any angular position of the crank arm 52 and 55, there exists a unique non-ambiguous position of the lever 24 and the bar 30.

I claim:

1. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion ofsaid link during an intermediate portion of the movement thereof in the course of a motion of 180 of said arm from one end dwell motion of said output member to another end dwell, that improvement which comprises:

a. secondary guide means on each side of said primary stabilizer guide,

b. means on said link to interengage said secondary guide means selectively near the respective ends of the reciprocal movement of said output member, and

c. means providing a slide connection between said arm and said link, wherein a controlled dwell of said lineal output member at the end of a stroke thereof is materially increased during rotation of said drive shaft.

2. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion ofthe movement thereof in the course of a motion of of said arm from one end dwell motion of said output member to another end dwell, that improvement which com prises: 1

a. a plurality of secondary guide means mounted adjacent said primary stabilizer guide positioned to lie adjacent the path ofa portion of said link at the respective ends of the reciprocal stroke of the output member,

b. follower means at said portion of said link to interengage said respective guide means to control the final progression of said link at the stroke extremities, and

c. slide connection in the form of a slot and pin on said drive arm and said link to enable said guide means to control the final motion of said link at said stroke extremities.

3. A drive as defined in claim 2 in which said guide means are positioned to engage said follower means as said stabilizer guide is disengaged from said link toward the end of a stroke.

4. A drive as defined in claim 2 in which said guide means are curved to control the acceleration and de celeration of said link at the end of the stroke.

5. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating linear movement, a drive arm driven by said shaft, a link connecting said arm and said output memher, and a primary stabilizer guide for one end portion of said link during an intermediate portion ofthe movement thereof in the course of a motion of 180" of said arm, an improvement to enhance the kinematic characteristics of the output member including a controllable modification of velocity and acceleration which comprises:

a. means on said arm and means on said link to provide a combination sliding and pivotal relation therebetween.

6. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180 of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises:

a. secondary guide means adjacent said primary stabilizer guide,

b. means on said link to engage said secondary guide means at the end of each reciprocal stroke, and

c. means on said arm and means on said link interengaging to permit a limited sliding motion and pivotal motion between said arm and said link.

7. In a trammel reciprocating drive of the type utilizing a dive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises:

a. a first means on said link engageable with said primary guide,

b. a second means on said link spaced from said first means,

c. secondary guide means above and spaced to each side of said primary guide positioned to engage with and guide said second means on said link respectively at respective ends of the movement of said link, and

d. means on said arm and means on said link interengaging to permit a limited sliding and pivotal motion between said arm and said link.

8. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180 of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises:

a. a means on said arm pivotally connected to said link to provide a combination sliding and pivotal relation therebetween at a point midway between the point of the axis of rotation of said arm and a pivotal connection to said output member, and

b. secondary guide means for said link operable at portions of the movement of the link when it is unguided by said stabilizer guide.

9. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, alink connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of of said arm from one end dwell motion of said output member to another end dwell, 'an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises:

a. a means on said arm pivotally connected to said link to provide a combination sliding and pivotal relation therebetween at a point spaced from the midpoint between the axis of rotation of said arm and a pivotal connection to said output member, and

b. secondary guide means for said link operable at portions of the movement of the link when it is unguided by'said stabilizer guide.

10. In a trammel reciprocating drive of the type utilzing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180 of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises:

a. means on said arm and means on said link to provide a combination sliding and pivotal relation therebetween,

b. a first means on said link engageable with said primary guide, I

c. a secondary guide means above and spaced to each side of said primary guide, and a secondary guide means spaced from said first means on said link, said secondary guide means being positioned to interengage and control the movement of said link respectively near the respective ends of the movement of said link. 

1. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, that improvement which comprises: a. secondary guide means on each side of said primary stabilizer guide, b. means on said link to interengage said secondary guide means selectively near the respective ends of the reciprocal movement of said output member, and c. means providing a slide connection between said arm and said link, wherein a controlled dwell of said lineal output member at the end of a stroke thereof is materially increased during rotation of said drive shaft.
 1. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, that improvement which comprises: a. secondary guide means on each side of said primary stabilizer guide, b. means on said link to interengage said secondary guide means selectively near the respective ends of the reciprocal movement of said output member, and c. means providing a slide connection between said arm and said link, wherein a controlled dwell of said lineal output member at the end of a stroke thereof is materially increased during rotation of said drive shaft.
 2. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, that improvement which comprises: a. a plurality of secondary guide means mounted adjacent said primary stabilizer guide positioned to lie adjacent the path of a portion of said link at the respective ends of the reciprocal stroke of the output member, b. follower means at said portion of said link to interengage said respective guide means to control the final progression of said link at the stroke extremities, and c. slide connection in the form of a slot and pin on said drive arm and said link to enable said guide means to control the final motion of said link at said stroke extremities.
 3. A drive as defined in claim 2 in which said guide means are positioned to engage said follower means as said stabilizer guide is disengaged from said link toward the end of a stroke.
 4. A drive as defined in claim 2 in which said guide means are curved to control the acceleration and deceleration of said link at the end of the stroke.
 5. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating linear movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm, an improvement to enhance the kinematic characteristics of the output member including a controllable modification of velocity and acceleration which comprises: a. means on said arm and means on said link to provide a combination sliding and pivotal relation therebetween.
 6. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises: a. secondary guide means adjacent said primary stabilizer guide, b. means on said link to engage said secondary guide means at the end of each reciprocal stroke, and c. means on said arm and means on said link interengaging to permit a limited sliding motion and pivotal motion between said arm and said link.
 7. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises: a. a first means on said link engageable with said primary guide, b. a second means on said link spaced from said first means, c. secondary guide means above and spaced to each side of said primary guide positioned to engage with and guide said second means on said link respectively at respective ends of the movement of said link, and d. means on said arm and means on said link interengaging to permit a limited sliding and pivotal motion between said arm and said link.
 8. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises: a. a means on said arm pivotally connected to said link to provide a combination sliding and pivotal relation therebetween at a point midway between the point of the axis of rotation of said arm and a pivotal connection to said output member, and b. secondary guide means for said link operable at portions of the movement of the link when it is unguided by said stabilizer guide.
 9. In a trammel reciprocating drive of the type utilizing a drive shaft, an output member mounted for reciprocating lineal movement, a drive arm driven by said shaft, a link connecting said arm and said output member, and a primary stabilizer guide for one end portion of said link during an intermediate portion of the movement thereof in the course of a motion of 180* of said arm from one end dwell motion of said output member to another end dwell, an improvement to enhance the kinematic characteristics including an increase in dwell and a controllable modification of velocity and acceleration which comprises: a. a means on said arm pivotally connected to said link to provide a combination sliding and pivotal relation therebetween at a point spaced from the midpoint between the axis of rotation of said arm and a pivotal connection to said output member, and b. secondary guide means for said link operable at portions of the movement of the link when it is unguided by said stabilizer guide. 